Model spectrum, in colour, over the observed data, in black, of AFGL 2591 for transition lines of (a) CH₃OH on the brightest pixel of the CH₃OH on the brightest pixel of the CH₃OH 6_0,6,0−5_0,5,0 (E_up = 48 K) line, and (b) H₂CO on the brightest pixel of the H₂CO 4_0,4−3_0,3 (E_up = 35 K) line. Three components are used to fit CH₃OH while one component is used to fit H₂CO. The dashed grey line represents the observed transition frequency, with the energy level of this transition labelled.